A Torso-Imaging System to Quantify the Deformity Associated With Scoliosis

1520 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 56, NO. 5, OCTOBER 2007

A Torso-Imaging System to Quantify the DeformityAssociated With Scoliosis

Peter O. Ajemba, Student Member, IEEE, Nelson G. Durdle, Doug L. Hill, and V. James Raso

Abstract—This paper describes a low-cost imaging systemthat is used to quantify the torso deformity that is associatedwith scoliosis. The system consists of image-capture and image-analysis components. The image-capture component obtains full-torso scans using a rotating positioning platform and one or twosurface digitizers. The image-analysis component assesses torsoshape. Results of system calibration and error analysis (accuracyof reproduction based on tests on a calibration box, a plaster cast,and a human subject) show that the system can be used to quantifythe deformity associated with scoliosis.

Index Terms—Scoliosis, shape monitoring, splines, surfacescans, torso imaging, torso models.

I. INTRODUCTION

S COLIOSIS is a condition involving a lateral curvature ofthe spine accompanied by rotation and twist of individual

vertebrae and visible torso asymmetries. Up to 70% of reportedcases are of unknown etiology and termed “idiopathic.” Al-though some cases of pulmonary dysfunction and low-backpain have been linked to severe scoliosis, many cases of lateonset idiopathic scoliosis will have no attendant health risksassociated with them [1]. Aside from improving the overallinternal alignment of the trunk, two primary goals of treatmentare to improve the external appearance of the torso and to haltthe progression of the deformity.

Traditional techniques that are used to describe the severityof scoliosis are based on assessing radiographs of the spine andcannot describe the visible torso deformities associated withscoliosis. Several researchers believe that torso imaging can de-scribe torso shape and also reduce the need for radiographs [2]effectively lessening the risk of cancer [3]. Several back-shape-imaging techniques have been proposed to assess scoliosis andmonitor its progression [4], [5]. These include the following:Moiré topography [6], integrated shape imaging system scan-ning [7], Quantec system scanning [8], rasterstereography [7],[9], and laser scanning [10]. Full-torso-imaging systems suchas Inspeck 3-D digitizers (Inspeck Corporation, Montreal, QC,Canada) and laser scanner-based systems [11] have also been

Manuscript received June 30, 2005; revised March 5, 2007.P. O. Ajemba and N. G. Durdle are with the Department of Electrical and

Computer Engineering, University of Alberta, Edmonton, AB T6G 2V4 Canada(e-mail: [email protected]).

D. L. Hill and V. J. Raso are with the Department of Rehabilitation Technol-ogy, Glenrose Rehabilitation Hospital, Edmonton, AB T5G 0B7 Canada.

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TIM.2007.903592

Fig. 1. Minolta 700 surface digitizer system and positioning platform.

proposed. These have been shown to provide more informationabout the overall asymmetry of the torso than back-surfacesystems [2].

However, most commercial systems are very expensive inequipment and labor as they require at least four scannersoperating in synchrony and several skilled operators to performtime-consuming postdata collection processing. Thus, they aregenerally unaffordable for scoliosis clinics operating on modestbudgets. Torso-surface imaging has been mostly used in scolio-sis clinics to attempt to predict the internal alignment of thespine from the external appearance of the torso [7], [8], [11]and to objectively track cosmetic changes of the back duringthe high-risk period of adolescence [10]. As the internal spinalalignment translates into the external torso shape through therib-cage, muscles, viscera, skin, and fat in a manner whichis peculiar to each patient and which changes over time asthe deformity progresses [2], this may not be a useful clinicalapplication [12].

Solving the problems of cost and utility requires developing away of upgrading existing back-shape-imaging systems to per-form full-torso imaging and finding a clinically relevant role for

0018-9456/$25.00 © 2007 IEEE

AJEMBA et al.: TORSO-IMAGING SYSTEM TO QUANTIFY THE DEFORMITY ASSOCIATED WITH SCOLIOSIS 1521

Fig. 2. Four positions of the digitizer.

torso imaging. These goals can be achieved by developing pro-tocols that are used to simultaneously or sequentially capturedifferent views of the torso, by ascertaining the errors that areassociated with forming full-torso scans from multiple partialscans, by determining the optimal operating conditions for thesystem based on existing technologies, and by developing waysto quantify and describe the external deformity associated withscoliosis.

In this paper, a low-cost torso-imaging system that is usedto quantify the deformity associated with scoliosis is presented.The system, an extension of a back-shape-imaging system incurrent use [10], has image-capture and image-analysis com-ponents. The image-capture component obtains full-torso scansusing a rotating positioning platform and one or two Minolta700 3-D surface digitizers (Konica Minolta Photo Imaging,Inc.) (Fig. 1). The image-analysis component is based onan AVS/Express (Advanced Visual Systems, Inc.) shell andassesses the shape of the torso based on the obtained torsoscans. The system architecture and possible configurations aredescribed. Results of system calibration and error analysis (ac-curacy of reproduction based on tests on inanimate objects andhuman subjects) are presented. The possible application of thesystem in quantifying and describing the deformity associatedwith scoliosis is discussed.

II. THEORETICAL BACKGROUND

A. Image Capture

A full-torso image can be obtained using a rotating position-ing platform and one or two surface digitizers.1) One-Digitizer Configuration: The use of the one-

digitizer configuration is based on the following assumptions:

four shots of an object taken at right-angles to each other willeffectively capture the full 360◦ view of the object; usingimage-matching techniques, it is possible to generate a full-torso scan from four orthogonal partial scans; and errors asso-ciated with obtaining partial scans of the torso and constructinga full-torso image are either acceptable or can be compen-sated for.

To obtain a full-torso image, four partial scans 90◦ apart aretaken by placing the digitizer in the four positions, which areshown in Fig. 2, relative to the scanned object. The digitizer isfixed and the rotating positioning frame is moved in a clockwisedirection (from position 1 through positions 2 and 3 to position4) and a partial scan is made at each position. The resultingclouds of points from each of the four positions are convertedinto mesh objects. These are stitched together to form full-torso scans using image-registration techniques. In regions ofoverlap between the meshes (typically, a 30% overlap existsbetween the mesh objects), a spline interpolation algorithm isused to produce the approximate surface. Further details of thistechnique can be found in [5].2) Two-Digitizer Configuration: In addition to the assump-

tions stated for the one-digitizer configuration, the use of thetwo-digitizer configuration is based on the assumption thaterrors due to patient repositioning (sway and breathing) andmotion artifacts [5] are reduced by averaging multiple full-torso scans to obtain an “average scan” [8]. The length of timebetween the initial and the final image is reduced by acquiringfull-torso scans with partial scans separated by at most onerotation compared to the four partial scans used in the singledigitizer configuration that are separated by up to three rotations(Fig. 3).

The number of unique arrangements of two digitizers in fourpositions around a circle is given by 4C2 = 2. These two unique


Fig. 3. Two arrangements for the two digitizer configuration.

arrangements are shown in Fig. 3. The two laser digitizers areplaced in positions 1 and 2 in arrangement 1 and in positions1 and 3 in arrangement 2. In both arrangements, four partialscans corresponding to the front, back, and sides of the scannedobject are needed in constructing a full-torso image.

B. Image Analysis

The torso model is based on 3-D spline-based mathematicalobjects—spline objects, and consists of a series of m latitudinalcross-sectional and k longitudinal cross-sectional closed splinecurves Sc and Sl, respectively, given by the following as:

Sc =m−1∑

j=0

ζcj (t) =

m−1∑

j=0

n−1∑

i=0

αci,j(t)Pi,j (1)

Sl =k−1∑

j=0

ζlj(t) =

k−1∑

j=0

n−1∑

i=0

αli,j(t)Pi,j (2)

where ζcj (t) and ζl

j(t) are the latitudinal cross-sectional andlongitudinal cross-sectional closed spline curves, respectively,Pi,j are the control points of spline curves i, and functionsαc

i,j and αli,j are spline basis functions. The numbers m and k,

and positions of the latitudinal cross-sectional and longitudinalcross-sectional spline curves used depend on the application athand. The longitudinal splines could be made to pass throughprominent anatomical landmarks on the torso, but in this paper,they are evenly spaced along the length of the torso. Thecontrol points Pi,j , may coincide with known landmarks on thelatitudinal and longitudinal cross-sectional splines.Definition 1: Let β represent a transformed version of a

spline object γ such that each of the closed spline curves inγ is transformed into an open spline curve in β by applyinga transformation T . Then, the object β made up of a seriesof modified spline curves is called a modified spline object. βcould be referred to as the “open equivalent” of spline object γ.

To quantify the shape of a torso, a spline object is generatedand transformed into an equivalent modified spline object.

Transformation T is used for this paper. T transforms eachcomponent spline curve of the spline object into an equiva-lent modified spline curve by plotting the distances from thecentroid of each spline curve to locations on the surface of thecurve as a function of the sector angles (formed by lines joiningthe centroid to the points on the surface) [Fig. 4(a)]. A plot ofsuccessive modified spline curves [Fig. 4(b)] yields a graph ofthe torso surface [Fig. 4(c)].

To compare two torso scans (A and B) of the same individualtaken at different times (or of two different individuals), splineobjects with equal numbers of horizontal closed spline curvesare obtained and transformed into their equivalent modifiedspline objects. The graphs obtained from the modified splineobjects are subtracted to produce a “difference” surface. The“difference” surface provides a quantitative and visual assess-ment of differences between the modeled torso surfaces.

III. MATERIALS AND METHODS

Scans of a test box (of dimensions 300 mm by 150 mm by200 mm; machined to an accuracy of 1 mm and measured toa precision of 0.1 mm) and a plaster cast (made from a castof an actual scoliotic torso) were used to assess the accuracyof the image-capture system. Analyses aimed at assessing theaccuracy of reproduction of the dimensions and the aspectratios of the sides of the test box, the effect of misalignmentof partial scans of the test box and the effect of using one ortwo digitizers on the accuracy of reconstruction of a plaster castwere performed.

To assess the accuracy of reproduction of the dimensions andthe aspect ratios of the test box, ten complete scans of the boxwere obtained using a single laser digitizer (Minolta VIVID700 Digitizer, Konica-Minolta, Inc., Mahwah, NJ) with the boxrepositioned after each scan. Five of these scans were takenwith the sides of the box perpendicular to the line of sight of thedigitizer. The other five were taken with the sides at an angle of45◦ to the line of sight of the digitizer. The lengths, widths,and heights of the reconstructed image of the test box were


Fig. 4. Obtaining a spline surface from a modified spline curve.

measured from each scan. The values of the three aspect ratiosof the test box (length–width, length–height, and height–width)were obtained from these values. The root-mean-square errorsand the standard deviations in the dimensions and the aspectratios of the test box were calculated. Errors here refer to thedifference between the measured and the actual values of thedimensions and the aspect ratios.

To assess the effect of aligning the partial scans of thetest box to an angle different from 90◦ on its reconstructionaccuracy, scans of the test box were obtained from partialscans aligned by 80◦, 82.5◦, 85◦, 87.5◦, 90◦, 92.5◦, 95◦, 97.5◦,and 100◦ from each other. The volumes of the models createdfrom misaligned partial scans of the test box were determined.The accuracy of reconstructing the models was assessed bycomparing their volumes to that of the actual box (300 × 150 ×200 mm3). In general, higher reconstruction accuracy impliedsmaller error in the volume of the reconstructed model. A plotof alignment error versus reconstruction accuracy was obtained.For any given value of reconstruction accuracy, a “maximumtolerable error of alignment” could be ascertained from the plot.The “maximum tolerable error of alignment” is the largest valueof alignment error that would produce a scan whose accuracy isless than or equal to the specified reconstruction accuracy.

To compare the effect of using one digitizer operating aloneor two digitizers operating in parallel on the accuracy of re-construction of a test object, ten scans of a plaster model ofa human torso were obtained using each of the two digitizersindividually. Ten other scans of the plaster cast were obtainedfrom the two digitizers at once (the digitizers were placedopposite each other as in arrangement 2 of Fig. 3). Ten evenlyspaced cross sections of each scan of the plaster cast wereobtained. The lengths of the scans were computed, and theirstandard deviations were calculated.

An integrated software package based on an AVS/Expressshell provided the visualization platform for the image-analysiscomponent of the system. Proprietary software developed by

our group and running as scripts off the shell performed taskssuch as computing cross sections of torso scans, calculatingpoints Pi,j [(1) and (2)], deriving orthogonal maps and com-puting difference maps from orthogonal maps. Difference mapsare usually obtained from two or more torso scans of differentpeople or of the same person taken at different times.

Two scans of the bare torso of a 22-year old male volun-teer who has no scoliosis or any other spinal deformity wereobtained within 1 h of each other. The volunteer freely movedaround between the scans. Coordinates of 40 longitudinal and40 longitudinal cross sections were computed from each scanand their corresponding control points (Pi,j) obtained. Twotorso models were created by computing series of latitudinaland longitudinal spline curves from each scan using thirddegree B-spline basis functions. The equivalent orthogonalsurfaces of the torso models were obtained. The two torsoscans were compared by subtracting their orthogonal surfacesto obtain a “difference surface” as shown in Section II-B. Theorthogonal and difference surfaces were analyzed to assess theglobal shape of the torso.

IV. RESULTS

Table I shows the root-mean-square errors and standard devi-ations in the dimensions of the sides of the test box. Overall, thedimensions of the test box varied by 1%–2%. The orientationof the test box with respect to the digitizer influenced itsdimensions. Scans obtained with the sides of the test box atan angle of 45◦ to the line of sight of the digitizer showedmore variation than scans obtained with the sides of the testbox perpendicular to the line of sight of the digitizer. Table IIshows the root-mean-square errors and the standard deviationsin the aspect ratios of the test box. Overall the aspect ratios ofthe test box varied by 1%–3%. The orientation of the test boxalso significantly influenced the variations in the aspect ratios ofthe scans obtained as higher variations were obtained for scans


TABLE IROOT-MEAN-SQUARE ERROR AND STANDARD DEVIATIONS IN THE DIMENSIONS OF THE TEST BOX

TABLE IIROOT-MEAN-SQUARE ERROR AND STANDARD DEVIATIONS IN THE ASPECT RATIOS OF THE TEST BOX

Fig. 5. Reconstruction accuracy as a function of the angle of alignment.

taken with the sides of the box at an angle of 45◦ to the line ofsight of the digitizer.

Fig. 5 shows a plot of the reconstruction accuracy (definedas the percentage difference between the volume of the re-constructed scan and the known volume of the test box) as afunction of the angle of alignment of the partial scans. For areconstruction accuracy of 5%, the “maximum tolerable error ofalignment” was found to be about 5◦. This implied that barringall other errors, the system could reconstruct an object to anaccuracy of at least 5% provided the partial scans of the objectwere set off by an angle between 85◦ and 95◦.

Table III shows the standard deviations in the values of thelengths of ten evenly spaced cross sections of the plaster castobtained using each of the two digitizers (digitizers A and B)alone and both digitizers simultaneously. The cross sectionswere numbered in increasing order from the waist of the castupwards. The standard deviations in the lengths of the first few

TABLE IIISTANDARD DEVIATIONS IN THE LENGTH OF CROSS-SECTIONS

OF A PLASTER CAST

cross sections (close to the waist of the cast) were lower thanthose of later cross sections due to persistent errors in aligningthe central axis of the cast to the line of sight of the digitizer.These errors were introduced by the motion of the test platformand are entirely a function of the setup of the experiment. Thestandard deviations in the lengths of cross sections obtainedfrom “single-digitizer scans” were lower than those obtainedfrom “double-digitizer scans.”

Two orthogonal surfaces were derived from the two scansof the male volunteer. Fig. 6 shows a plot of one of the


Fig. 6. Orthogonal surface obtained from torso scans of a male volunteer. The surface was divided into five sections: 1: chest; 2: stomach; 3: right scapula;4: left scapula; 5: back.

orthogonal surfaces. The orthogonal surfaces and a differencesurface (obtained by subtracting the two orthogonal surfaces)were divided into five regions: chest, stomach, right scapula,left scapula, and back for ease of analysis (Fig. 6). Each of theregions in the difference scan were analyzed independently forvariations and showed a maximum variation of less than 4 mmbetween the two scans.

V. DISCUSSION AND CONCLUSION

A full-torso-imaging system using one or two MinoltaVIVID 700 digitizers and a rotating positioning platform wasdescribed. The system consisted of image-capture and image-analysis components and required four partial scans of an objectset off by 90◦ to produce a full scan. Several models of a testbox were obtained to assess the reconstruction accuracy of thesystem’s image-capture component. The dimensions and theaspect ratios of the models of the test box obtained varied byless than 3%. This was comparable to results obtained fromour previous work [5]. In [5], the maximum variations in thedimensions of models obtained from five volunteers were foundto be less than 4%. The higher error value was attributed toerrors caused by sway and breathing.

Several models of a plaster cast of an actual scoliotic torsowere obtained and analyzed to quantitatively compare theimage-capture component of the one-digitizer configurationto that of the two-digitizer configuration. As expected, errorsassociated with the two-digitizer configuration were lower thanthose associated with the one-digitizer configuration. To assessthe effect of misaligning, the four partial scans required toobtain a full-torso model on the accuracy of reproduction of thesystem, models of the test box were created from partial scanswith varying degrees of misalignment and analyzed. As thedesign of the system almost certainly precludes the possibility

of misaligning the partial scans by up to 5◦, the maximum errorof alignment of the system was found to be 2%.

To assess the output of the image-analysis component of thesystem, two orthogonal surfaces derived from two full-torsoscans of a male volunteer were obtained (one of which is shownin Fig. 6). A difference surface was obtained by subtractingthe two orthogonal surfaces. The orthogonal and differencesurfaces were divided into five sections and analysis showedthat the peak height differences in each of the sections of thedifference surface were less than 4 mm. Most of the point-by-point differences observed were attributable to errors causedby patient repositioning and sway and are less than the size ofanatomical landmarks on the torso.

These results indicate that the image-capture component ofthe system can be used to create acceptable models of the torso.They also indicate that the image-analysis component of thesystem can be used to assess the shape of the torso. As theoutput of the system is quantitative, it may be possible to applythe system to quantifying and describing shapes. One possibleapplication is to quantify and describe the deformity associ-ated with scoliosis from analysis of torso scans of scolioticpatients.

A limitation of the system is that two fairly skilled people areneeded for its operation. Future work will focus on automatingthe operation of the rotating positioning platform and on im-proving the image-analysis software to reduce the number ofskilled operators needed to just one.

REFERENCES

[1] R. A. Dickson, “Spinal deformity—Adolescent idiopathic scoliosis: Non-operative treatment,” Spine, vol. 24, no. 24, pp. 2601–2606, Dec. 1999.

[2] J. L. Jaremko, P. Poncet, J. Ronsky, J. Harder, J. Dansereau, H. Labelle,and R. F. Zernicke, “Estimation of spinal deformity in scoliosis fromtorso surface cross sections,” Spine, vol. 26, no. 14, pp. 1583–1591,Jul. 2001.


[3] A. R. Levy, M. S. Goldberg, N. E. Mayo, J. A. Hanley, and B. Poitras,“Reducing the lifetime risk of cancer from spinal radiographs among peo-ple with adolescent idiopathic scoliosis,” Spine, vol. 21, no. 13, pp. 1540–1548, Jul. 1996.

[4] P. O. Ajemba, N. G. Durdle, D. L. Hill, and V. J. Raso, “Effect ofposture on a full torso imaging system for the assessment of scoliosis,”in Proc. Conf. Int. Res. Soc. Spinal Deformities, Vancouver, BC, Canada,Jun. 10–12, 2004.

[5] P. O. Ajemba, N. G. Durdle, D. L. Hill, and V. J. Raso, “Re-positioningeffects on a full torso imaging system for the assessment of scoliosis,” inProc. IEEE Can. Conf. Elect. Comput. Eng., Niagara Falls, ON, Canada,May 2–5, 2004, pp. 1483–1486.

[6] M. S. Moreland, M. H. Pope, D. G. Wilder, I. A. Stokes, andJ. W. Frymoyer, “Moire fringe topography of the human body,” Med.Instrum., vol. 15, no. 2, pp. 129–132, Mar./Apr. 1981.

[7] T. N. Theologis, J. C. Fairbank, A. R. Turner-Smith, andT. Pantazopoulos, “Early detection of progression in adolescent idiopathicscoliosis by measurement of changes in back shape with the integratedshape imaging system scanner,” Spine, vol. 22, no. 11, pp. 1223–1227,Jun. 1997.

[8] C. J. Goldberg, E. E. Fogarty, D. P. Moore, and F. E. Dowling, “Scoliosisimaging and the problem of postural sway,” in Research Into SpinalDeformities 1: Series Studies in Health Technology and Informatics,J. A. Sevastik and M. D. Khaled, Eds. Oxford, U. K.: IOS, 1997.

[9] L. Hackenberg, E. Hierholzer, W. Potzl, C. Gotze, and U. Liljenqvist,“Rasterstereographic back shape analysis in idiopathic scoliosis after an-terior correction and fusion,” Clin. Biomech., vol. 18, no. 1, pp. 1–8,Jan. 2003.

[10] D. L. Hill, D. C. Berg, V. J. Raso, E. Lou, N. G. Durdle, J. K. Mahood, andM. J. Moreau, “Evaluation of a laser scanner for surface topography,” inResearch Into Spinal Deformities 3: Series Studies in Health Technologyand Informatics, A. Peuchot, Ed. Oxford, U. K.: IOS, 2003.

[11] P. Poncet, S. Delorme, J. L. Ronsky, J. Dansereau, G. Clynch, J. Harder,R. D. Dewar, H. Labelle, P. Gu, and R. F. Zernicke, “Reconstruction oflaser-scanned 3D torso topography and stereoradiographical spine and rib-cage geometry in scoliosis,” Comput. Methods Biomech. Biomed. Eng.,vol. 4, no. 1, pp. 59–75, 2000.

[12] I. A. F. Stokes, “Point of view: Estimation of spinal deformity in scoliosisfrom torso surface cross sections,” Spine, vol. 26, no. 14, pp. 1583–1591,Jul. 2001.

Peter O. Ajemba (S’04) received the B.Eng. de-gree (first-class honors) in electrical and electronicsengineering from the University of Benin, Benin,Nigeria, in 2000. He is currently working toward thePh.D. degree in electrical and computer engineeringat the University of Alberta, Edmonton, AB, Canada.

His research work is aimed at developing image-analysis and biomedical informatics techniques toaid surgeons in managing scoliosis and assessingtorso asymmetries. He is currently working on anovel curvature-based shape model that is used toassess and track torso deformity in scoliosis.

Nelson G. Durdle received the Ph.D. degree in com-puter engineering from the University of Alberta,Edmonton, AB, Canada, in 1982.

He is a Professor with the Department of Electricaland Computer Engineering, University of Alberta.He has more than 20 years of experience in the areasof biomedical engineering and medical applicationsof computers. Currently, he maintains research lab-oratories at both the University of Alberta and atGlenrose Rehabilitation Hospital, Edmonton, wherehe has a long-standing collaborative relationshipwith several practicing physicians.

Doug L. Hill received the B.Sc. degree in computerengineering and the MBA degree from the Universityof Alberta, Edmonton, AB, Canada, in 1984 and1992, respectively.

He is a Clinical Engineer with Glenrose Rehabil-itation Hospital, Edmonton, and is interested in theclinical assessment of spinal deformities.

V. James Raso received the B.Sc. and M.Sc. degreesfrom the University of Waterloo, Waterloo, ON,Canada, in 1975 and 1977, respectively.

He is Head of the Orthopaedic BioengineeringGroup, Glenrose Rehabilitation Hospital, part of theCapital Health Authority, Edmonton, AB, Canada.He has had a long-standing interest in the cause andtreatment of spinal curvatures in children.

Mr. Raso is an active member of both the Scolio-sis Research Society and the International ResearchSociety of Spinal Deformity.

A Torso-Imaging System to Quantify the Deformity Associated With Scoliosis

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